Hi Everyone I was trying to do a young's double slit experiment with green laser. The gap between the slits was d = 0.001m The distance to the screen was D =1.2m We counted from the center 10 bright fringes m = 10 The distance of the from the center bright fringe was y = 0.067 m Using a small angle approximation: y = (mλD)/d So λ = (yd)/mD = (0.067 x 0.001)/12 = 5.5 x 10^-6 Wavelength of laser given as about 550 nm = 5.7 x 10^-7 ouch! out by a factor of ten. Can't figure out what I did wrong. NB: This is not a homework question. I am just trying to make the experiment work, and though I am ashamed of my incompetence, I am actually the teacher! Thanks in advance for any suggestions. The bright spot in the middle was a bit big and smudged. But that was partly why we chose to count as many as 10 fringes. Even if for some reason there were an extra one or two fringes that got burnt out at the center, that would not account the factor of ten.